1. FIELD OF THE INVENTION
The invention relates to safety valves used in subterranean wells, such as oil or gas wells, and specifically to valves which are actuated in response to changes in control fluid pressure in a separate control line extending from the subsurface valve location to the well surface.
2. DESCRIPTION OF THE PRIOR ART
Subsurface safety valves are commonly used in oil or gas wells to prevent the escape of fluids from a producing formation in the event of damage at the well surface. Typically, these valves are incorporated into the fluid transmission production tubing which is inserted through the well casing and extends from the surface of the well to the producing formation. The flow of fluids through this inner tubing string must be interrupted in the event of damage to the well head at the surface. By positioning these valves at a location below the well surface, for example, below the mud line in an offshore well, the safety valve can be closed to prevent the escape of produced fluids.
The most common subsurface safety valves employ either a shiftable flapper or a rotatable ball valve head to open and close the fluid transmission conduit. Commonly, either a shiftable flapper or a ball valve head is actuated from closed to an open position in response to axial movement of an actuator sleeve. In conventional ball valves, downward movement of the actuator sleeve will impart rotation to a closed ball valve head to rotate a central passage through the ball into alignment with the fluid transmission conduit. In conventional flapper valves, the actuator sleeve will engage a closed flapper to shift the flapper to its open position by rotation in a vertical plane about the flapper hinge. In these conventional valves, axial movement of the actuator sleeve is normally imparted by an increase in control fluid pressure. Hydraulic control fluid is generally supplied through an external control line extending from the valve to the well surface. An increase in pressure can be initiated at the well surface and this increase in pressure is transmitted through the hydraulic control fluid to the downhole valve. Conventionally, unbalanced piston surfaces on the actuator sleeve are exposed to the control pressure and an increase in control pressure will result in an increase in the control force acting on the actuator sleeve. Eventually a sufficient pressure force will be created to urge the actuator sleeve downwardly relative to the closed flapper or ball valve head against the action of the well pressure and any spring which might be used to hold the actuator sleeve and the valve head in the closed position. When control fluid pressure is reduced or removed, for any reason, the well pressure and the spring acting on the actuator sleeve will be sufficient to shift the actuator sleeve upwardly permitting the valve head to close.
In addition to these conventionally actuated valves, some subsurface safety valves employ shiftable spools or pistons for actuating a flow tube actuator. These separate shiftable spools or pistons may be employed to reduce the surface area upon which the ambient well fluid pressure acts. A reduction in surface area means that the hydraulic pressure force opposing the shifting of the actuator sleeve to a valve-opening position is less, thus permitting the safety valve to be positioned at greater depths. U.S. Pat. Nos. 4,005,751, 4,119,146, 4,161,219, and 4,503,913 each disclose subsurface safety valves having a spool or piston mounted in the valve housing and operatively connected to the valve-actuator sleeve.
Occasionally, a defect or leak in the valve spool or piston will prevent the development of sufficient force to shift the actuator sleeve to a valve-opening position. In such case, the availability of a backup hydraulic system including a separate hydraulic line extending to the well surface is a desirable adjunct. More importantly, if the primary piston is not operable, it becomes immediately desirable that the safety valve be shifted to, and locked in, an open position to ensure that wireline tools may be inserted through the safety valve to effect the control of other tools located downhole relative to the safety valve.
Accordingly, it is highly desirable that safety valves operated by a small-diameter piston be provided with a backup hydraulic mechanism for not only effecting the movement of the safety valve from a closed to an open position, but also for effecting the locking of the safety valve in such open position. An economical, yet reliable, combination backup hydraulic mechanism and locking mechanism has not heretofore been provided in the prior art.